1. Field of the Invention
The present invention relates generally to improvements in rotating disk magnetic data storage devices, and, more particularly, but not by way of limitation to improvements in locating data sectors on the disks of such devices.
2. Brief Description of the Prior Art
In rotating disk magnetic data storage devices, data is stored in sectors extending angularly along concentric data tracks defined on the disks of the device. The disks have magnetizable surface coatings. Data is written and subsequently read by transducer heads that fly over the surfaces of the disks to magnetize cells of the surface coating, for writing, or respond to differences in magnetization of adjacent cells for reading. Both operations are controlled by a read/write controller that provides encoded data to the transducer head during writing and receives emf pulses from the transducer heads during readback of the data.
For such a system to operate, it is necessary for the sectors to be located prior to reading or writing and it is common practice to encode a data sector with a header that identifies the sector. Some means must be provided to supply sector location pulses to the controller to enable reading of information on the track as the transducer head approaches alignment with the header. Once the appropriate sector has been reached, reading or writing of data from or to the disk can proceed.
In the past, it has been common practice to include address marks on the disks that violate the code used in writing the data and the headers. A circuit can then be constructed to search for the address marks which the controller will place at the beginning of the sector. Such a circuit then provides the "sector" location pulses to the read/write controller.
The use of address marks on a disk suffers from the disadvantage that the marks can be lost for any of a number of reasons; for example, through flaws in the magnetic medium in which the data is written or accidental turn on of a write gate, used to enable writing, as a transducer head passes over an address mark. In this case, the data stored in the sector for that address mark has been lost. That sector's data can never be retrieved because the controller will never receive the pulses necessary for locating the sector. Similarly, read errors while searching for the address mark may cause a sector to be missed and lower the throughput of the data storage device.
The highly preferred alternative has been for the disk storage device to output sector location pulses at the required regular interval without having to write or recover any special data on the disk media itself. This is usually done with a simple circuit that counts out desired time (or number of bytes) in a sector before issuing the next sector location pulse. This simple circuit has been referred to as hard sectoring. Simple hard sectoring has proven adequate for decades of years because the time when sector location pulses should occur has been identical on every track of the disk storage device.
The problem is exacerbated by other requirements placed on a rotating disk data storage system. As is well known, it is desirable to store as much data on a disk as possible and this desire has lead to the recording of data at different frequencies on different tracks of the disk as taught by Bremmer et al. in U.S. Pat. No. 4,799,112 issued Jan. 17, 1989, the teachings of which are hereby incorporated by reference. With recording of different tracks at different frequencies, sectors on different tracks occupy different angular lengths that take differing times to pass by a transducer head. Accordingly, for rotating disk data storage devices that utilize different data transfer rates for different tracks, sector location pulses must be supplied to the controller at different rates that depend upon the radial location of the transducer head on the disk. As a result, it has been necessary in the past to either forego recording tracks at different frequencies or use address marks, despite the disadvantage of much lower data security.